The High-Redshift Quasar Luminosity Function from Multi-Epoch Imaging Surveys
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Upcoming time-domain imaging surveys such as the LSST will detect over a million high-redshift z > 4 quasars, making complete spectroscopic followup unfeasible. Statistical estimates such as luminosity functions and clustering measurements will require purely photometric methods for classifying quasars, estimating redshifts and estimating selection functions. We validate these methods and constrain the optical, type I quasar luminosity function (QLF) at 3.75 < z < 4.5 for −27.5 < M_1450 < −23.5. This thesis presents the first variability-selected QLF measurement at high redshift (z > 3.75) and constraint on the characteristic luminosity (M* = −26.7) from a single, uniformly-selected survey at z ~ 4. We used the Sloan Digital Sky Survey (SDSS) repeated imaging of the 275 sq. deg. equatorial region of the sky (−50 < R.A. < +60; −1.26 < Dec. < +1.26), known as Stripe 82, to select a statistical sample of z ~ 4 quasars. We extracted 40 million lightcurves from the imaging using forced photometry on all u, g, r, i, z epochs at the positions of sources detected on a deep i-band co-add. We developed a classification method based on photometric information alone (colors and variability metrics derived from these new multi-band lightcurves), which we validated with a spectroscopically complete 55 sq. deg. sub-region augmented with 102 new spectroscopic observations of quasars at z > 3.4 with i < 22.5. We demonstrate that selection functions for ensemble classifiers can be estimated by building generative models of empirical distributions of quasars previously selected with a diverse set of selection criteria. The z ~ 4 QLF contributes to our understanding of supermassive black hole growth and cosmic reionization of both H I and He II which likely began at z ~ 4 as a result of hard UV emissivity from quasars. The resulting QLF measurement is consistent with the previous lower number densities reported from deep, narrow-field surveys (COSMOS); it is not consistent with higher number densities reported from the NDWFS-DLS and CANDELS GOODS-S fields. In the context of recent 2.2 < z < 5 estimates from SDSS-III BOSS and Stripe 82, it indicates that the QLF evolves in both luminosity and density: the characteristic luminosity increases and density decreases log-linearly from z = 2.2-->5. It predicts that quasars contribute 100% of the ionizing emissivity needed to balance hydrogen recombinations at z ~ 4. Given that at z ∼ 6 hydrogen recombines faster and quasars are less abundant, this result suggests that quasars contributed a negligible fraction of UV photons to H I reionization. Finally, we find that the number of quasars at z ~ 4 is sufficient to ionize He II by z ~ 3, consistent with independent observations of the He II Lyman-alpha forest and temperature of the intergalactic medium.
- Astronomy